Hess

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Hesss Law
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Definition of Hesss Law

• If a reaction is carried out in a series of
  steps, ?H is the sum of all the enthalpy changes
• CH4(g) O2(g) ? CO2(g) 2H2O(g) ?H-802
• 2H2O(g) ? 2H2O(l) ?H-88
• --------------------------------------------------
  ----------
• CH4(g) O2(g) ? CO2(g) H2O(l) ?H-890

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Example

• The heat of combustion of C to CO2 is 393.5
  kJ/mole and the heat of combustion of CO to CO2
  is 283.0 kJ/mole. Calculate the heat of
  combustion of C to CO.

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Example 2

• Carbon occurs in two forms graphite and diamond.
  The heats of combustion are shown
• C(graphite) O2 ? CO2 ?H -393.5kJ
• C(diamond) O2 ? CO2 ?H -395.4kJ
• Calculate the ?H for the conversion from
  graphite to diamond.

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Example 3

• Calculate the ?H for
• 2C(s) H2(g) ? C2H2(g)
  given that
  
• C2H2(g) 2.5O2 ? 2CO2 H2O(l) ?H-1299.6
• C O2 ? CO2 ?H-393.5
• H2(g) .5O2(g) ? H2O(l) ?H-285.9

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Whats It Mean Practically

• Can never obtain more or less energy form a
  chemical reaction by changing the method or
  number of steps in the reaction